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Tobu's Series 5000 memo >> Tobu 5000: Nose suspension drive
Page first written 18 June 2003
Last updated 18 June 2003

Nose-suspension drives

Tobu's Series 5000 memo

The nose-suspension drive is the drive mechanism for electric railcars, in which approximately half of the weight of the traction motor is supported by the wheel axle through the axle metal, and the rest by the bogie frame through the "nose" on the motor frame. The axle metal does not allow any relative movements between the wheel axle and the motor axle (except for a very limited allowance), which makes it possible to directly connect the two axles by a gearbox.

In the so-called Cardan drive, mechanisms such as "Cardan shafts" will allow some amount of relative movements. In principle, the axle metal will not allow such relative movements, which will eliminate the need for such a mechanism. However, the weight of the motor supported through the axle metal will be "the unsprung mass", which will have higher impact on the track destruction than the sprung mass. In addition, the vibration that comes directly from the axle to the motor is said to contribute to slower allowable rotating speed of the motor, resulting in the increase of the motor mass. There seems to be various side effects caused by this vibration, and the axle metal has got a "room" for small relative movements of the motor and wheel axles. This allowance results in the noise, affectionately called "nose-suspension sound" by Japanese rail enthusiasts, which comes from the gear. These apparent drawbacks resulted in the decline of nose suspension drives in Japan, where their use in newly built railcars has ceased with only a few exceptions (including Tobu Railway) since the end of 1950s.

In 1950s, not only the move from nose-suspension drive to Cardan drive, but various improvements, such as bogie dynamics, car body weight reduction or the riding comfort, took place at the same time. This is why, in Japan, emus with nose suspension drives were effectively the outdated stock. However, this is not necessarily true. For example, Class 321 in the UK, introduced in 1988, can actually run at the top speed of 100mph (160km/h), with only one car out of 4-car trainset equipped with traction motors and nose-suspension drives. Apart from the gear noise, the emus pose no problem at all. In London, the Chancery Lane derailment that took place in January 2003 was said to be due to the improperly maintained WN coupling, an essential part of the Cardan drive of the fleet -- which seemed to me a reflection of the fact that nose suspension drives, with its simplicity and reliability, were not at all outdated here in the UK. In Japan, too, on the 762mm narrow gauge line owned by Kintetsu (Kinki Nippon Railway), the trainsets introduced in the 1980s came with nose suspension drives because of insufficient space inside the back gauge of the wheelset, although the traction motors were of modern design and lightweight. Another interesting example in Japan should be the newest electric locomotives now being introduced by JR Freight (except Class EF200). This fact, which I believe is so out-of-date, is in good contrast with many examples in continental Europe, where carbody-mounted motors are commonplace for the locomotives (most Japanese electric railcars with Cardan drives come with bogie-frame-mounted motors).

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(c) R. Takagi 2003. All rights reserved.